The present invention relates to a luminescent device and more particularly, to an electroluminescence (which will be referred to merely as EL, hereinafter) device which light wavelength is variable and which can be suitably used for optical logical devices, display devices, communication luminescent devices, read/write heads for information files, printers, sensors, etc. in information communication fields.
There is disclosed in a journal entitled "Japanese Journal of Applied Physics", vol. 27, No. 2 (1988), pp., L269-L271 that an emission spectrum is obtained based on a molecular formula for respective organic phosphor materials of a thin film.
FIG. 2 is a cross-sectional view of an ordinary organic luminescent device which uses the aforementioned thin film of the organic phosphor materials. The organic luminescent device comprises a glass substrate 101, electrically conductive transparent electrode films 103 formed on the glass substrate 101, a hole injection layer 104 made of diamine derivative (abbreviated to TAD), an active layer 105, metallic electrodes 106, these layers being sequentially formed in this order.
The electrically conductive transparent electrode films 103 and metallic electrodes 106 are arranged to be orthogonal to each other, in a matrix form. When a D.C. voltage of 5-20 V is applied to the matrix with the electrically conductive transparent electrode films 103 as a plus terminal and the metallic electrodes 106 as a minus terminal, an intersection part between the plus film and minus electrode emits light that exits from the glass substrate 101. The light emission part is called a pixel. The emission spectrum of the matrix is determined by the type of the luminescent material. When aluminum chelate (abbreviated to ALQ) is employed as the luminescent material, such a broad emission spectrum as shown in FIG. 3 is obtained. In this case, the emission spectrum has been determined essentially uniquely. Further, when it is desired to modify the emission spectrum, it has been common practice to dope a color filter but been impossible to freely modify the emission spectrum of a once-prepared device.
Also disclosed in a magazine entitled "Appl. Phys. lett.", Vol. 63(5), No. 2, August 1993 is such a matrix that, as shown in FIG. 8 (in the present application), comprises mirror electrodes of organic thin films formed for the metallic electrodes 106 in FIG. 2, a semi-transparent reflective film (half mirror) 102 formed on a glass substrate 101 to have a multi-layered structure of TiO.sub.2 and S.sub.i O.sub.2 films, electrically conductive transparent electrode films 103 formed on the semi-transparent reflective film 102, a hole injection layer 104 of diamine derivative (TAD) formed on the electrically conductive transparent electrode films 103, an active layer 105 of aluminum chelate (ALQ) formed on the hole injection layer 104, metallic electrodes 106 formed on the active layer 105, these films and layers being formed in this order, so that the emission intensity of the organic thin films is increased based on the cavity effect to light between the metallic electrodes 106 and semi-transparent reflective film 102 and at the same time, the emission spectrum of the matrix is made narrow.
In other words, the above citation reports that, when a distance between the metallic electrodes 106 and semi-transparent reflective film 102 is set to correspond to a specific wavelength in the emission spectrum range (between 450 nm and 700 nm) of the active layer 105, light having the specific wavelength is resonated to increase the intensity of the light as shown in FIG. 9.
There is disclosed in U.S. Pat. No. 5,003,221 a liquid crystal device which comprises liquid crystal elements made up of a transparent substrate 11, stripe electrodes 13, a dielectric layer 14, an EL layer 15, a dielectric layer 16, stripe electrodes 17 formed sequentially on the transparent substrate 11, and a thin film layer 12 between liquid crystal elements, wherein the diffraction index of the thin film layer 12 is set so that a difference in the diffraction index between the transparent substrate 11 and liquid crystal elements becomes a minimum, thus reducing reflection of light coming from the outside to the liquid crystal device.
However, in the above prior art, it has been not possible to change the wavelength of the emission spectrum of the luminescent device.
Meanwhile, JP-A-3-197923 discloses a liquid crystal device in which at least two liquid crystal layers having a birefringence index varied by a voltage are provided between an analyzer and a polarizer and the birefringence is controlled by the voltage between liquid crystal electrodes to utilize the multiple liquid crystal layers as a variable color filter. However, this liquid crystal device becomes large in size when compared with the aforementioned thin film EL device, because a light source is required to be provided outside of the liquid crystal device.
The above prior art has had a problem that since the color of light issued from the luminescent device is determined uniquely by the used luminescent material, the color of the emitted light cannot be controlled.
That is, in the method disclosed in the above magazine "Appl. Phys. lett.", it is disadvantageously impossible to control the emission spectrum by an input signal.
The above JP-A-3-197923 has had problems that since the multi-layered variable color filter, the light transmission loss is large and that since a light source must be provided outside of the liquid crystal elements, the overall device becomes large in size.